In a vehicles, such as vans and tracks, which aim to carry humans and burdens, a movable load changes depending on an amount or a weight of a load. In the case where a damping force characteristics of a shock absorber incorporated in a suspension is constant with such vehicle, a ride comfort in the vehicle changes depending on the weight of the movable load. For example, when setting the damping force characteristics of the shock absorber to be optimal in the case of heavy movable load, if the movable load is light, the damping force becomes excessive, resulting in bad ride comfort in the vehicle. In contrast, when setting the damping force characteristics of the shock absorber to be optimal in the case of light movable load, if the movable load is heavy, the damping force becomes insufficient, resulting in bad ride comfort in the vehicle.
Therefore, there has been developed a shock absorber that automatically changes the damping force characteristics sympathizing with the movable load. Such shock absorber includes, for example, a cylinder, a piston, a piston rod, an expansion-side port, a compression-side port, an expansion-side leaf valve, a compression-side leaf valve, piston nuts, bypass paths, a shutter, springs, and control springs. The piston is slidably inserted in the cylinder to partition the inside of the cylinder with an expansion-side chamber and a compression-side chamber. The piston rod is movably inserted into the cylinder and is coupled to the piston. The expansion-side port and the compression-side port are disposed at the piston. The expansion-side leaf valve opens and closes the expansion-side port. The compression-side leaf valve opens and closes the compression-side port. The piston nuts are mounted to the distal end of the piston rod. The bypass paths open from the side portions of the piston nuts to communicate between the expansion-side chamber and the compression-side chamber via the piston rods. The shutter is slidably mounted to the outer periphery of the piston nut to open and close the bypass paths. The springs bias the shutter to the direction of opening the bypass paths. One ends of the control springs are secured to the cylinder while the other ends are opposed to the shutter.
In the case of light movable load with the above-described shock absorber, since the shutter does not abut against the control springs and the bypass paths are kept to be open, the shock absorber outputs a low damping force. In contrast to this, in the case of heavy movable load, the position of the piston sinks downward, and the control springs push the shutter to obstruct the bypass paths. Thus, the shock absorber outputs the high damping force. Thus, the above-described shock absorber can automatically adjust the damping force characteristics according to the movable load (for example, see JP2000-225823A).